Pigments comprising oriented three-dimensionally crosslinked substances having a liquid-crystalline structure with a chiral phase are known, for example, from DE 42 407 43 (corresponding to U.S. Pat. No. 5,362,315). These pigments have a color which is dependent on the viewing angle and can be employed in any desired media, such as coating materials, binders or plastics, for example. This is described at length in, for example, EP 0 686 674. A broad field of application is covered by such pigments in connection with the painting of articles such as motor vehicles, for example.
As noted in EP 0 724 005, pigments prepared in accordance with DE 42 40 743 exhibit changes in color if the paint comprising said pigments is processed at different stoving temperatures. This is the case, for example, with motor vehicle finishes.
In accordance with the state of the art, motor vehicles are OEM finished at 130.degree. C., while in the case of subsequent refinishes just 80.degree. C. is tolerated, since constructional elements of the motor vehicle can be damaged at higher temperatures. The color differences between the original finish and the refinish, resulting from the different stoving temperatures, are disadvantageous in that they can be recognized with the naked eye.
In order to solve this problem EP 0 724 005 A2 proposes crosslinking the starting substances specified in DE 42 40 743 for preparing the pigments together with further, color-neutral compounds comprising at least two crosslinkable double bonds. The intention of this is to raise the crosslinking density of the pigments, which is said to lead to greater color stabilities in the case of application in a 130.degree. C. and an 80.degree. C. motor vehicle paint system.
EP 0 724 005 A2 describes pigments prepared in this way which instead of the shift in the wavelength maximum which is commonly observed with the pigments prepared in accordance with DE 42 40 743 feature a shift of just 17-24 nm when comparing room temperature drying of the refinish versus stoving temperature at 130.degree. C. and, at a temperature differential established between 80.degree. C. and 130.degree. C. drying temperatures, just 10-14 nm instead of the customary 20-25 nm. Consequently, by modifying the composition as described, EP 0 724 005 has reduced the wavelength difference by half, but by no means to values of a few nm. It is only at such low wavelength differences that--depending on the spectral sensitivity of the human eye--color differences can no longer be perceived visually. The problem of the deficient color stability of the pigment-containing compositions at different application temperatures therefore continues to exist.